This invention pertains to a process for producing oxidized organic chemical products by selective catalytic oxidation of organic chemical feedstocks with hydrogen peroxide intermediate. It pertains particularly to such a selective oxidation process in which the hydrogen peroxide intermediate is provided by direct reaction of hydrogen and oxygen using a supported phase-controlled noble metal catalyst having high reactivity and selectivity.
Selective oxidation reactions are a major class of chemical transformations which account for the production of a wide variety of important chemical products, including alcohols, carbonyl compounds, epoxides, hydroxylates, acids, glycols and glycol ethers, lactones, oximes, and oxygenated sulfur and nitrogen compounds such as sulfoxides, sulfones, nitrones, azo compounds, and other N-oxides. Performing these chemical transformations efficiently, economically, and safely requires a suitable oxidizing agent which can be purchased or produced to react with the desired organic chemical feedstock, which is then converted to the oxidized organic chemical product.
Several significant problems face conventional catalytic oxidation processes. In some cases, selective oxidation reactions are performed utilizing oxygen as the oxidizing agent. However, producing purified oxygen is expensive, requiring large capital investment and operating costs. Also, processes using purified oxygen combined with organic chemical feedstocks may accidentally achieve gas compositions in the explosive range, thereby posing a serious safety hazard. In other cases, selective oxidation processes utilize air as the oxidizing agent. But a major economic problem associated with such processes utilizing air is handling the accompanying undesired large flow of nitrogen, which substantially increases process costs. Such oxidation processes can also be prone to forming explosive gas mixtures. Oxidative processes using oxygen or air also tend to suffer from product selectivity problems related to overoxidation of the organic chemical feedstock, normally producing undesired carbon oxides (CO, CO2).
For such selective oxidation reactions, an attractive alternative to using oxygen or air as the oxidation agent is the use of peroxidic compounds to provide the reactive oxygen needed for oxidative transformations. One common version is the use of organic hydroperoxides as oxidizing agents. These hydroperoxide compounds, typically generated by air- or O2-oxidation of suitable intermediates, are reacted with chemical feedstocks to form oxygenated products and organic by-products. However, these organic by-products represent a significant disadvantage for processes of this type, because a large amount of organic material must be recovered, either for recycle or for sale as a secondary product. In some cases, the amount of this secondary product is greater than the amount of the primary oxygenated product, and is typically a less desirable product. For example, conventional production of propylene oxide also results in production of large amounts of styrene or tert-butyl alcohol co-products, which typically must be marketed in economically unpredictable markets. Furthermore, processes involving organic hydroperoxide intermediates pose significant safety hazards. Generating hydroperoxides requires reactions of air with organic chemicals which may form explosive mixtures, and organic peroxides can themselves be explosive, particularly if they are accidentally concentrated above a certain critical concentration level.
Instead of using organic peroxides, hydrogen peroxide is a known desirable oxidizing agent. The byproduct of oxidation reactions using hydrogen peroxide is typically water, a safe compound that can be easily recovered and reused or disposed. The amount of water on a weight basis is much less than the amount of organic by-product when organic hydroperoxides are used, and thereby represents significant savings in process costs. However, past attempts to develop selective chemical oxidation processes based on hydrogen peroxide have encountered significant difficulties. Conventional hydrogen peroxide production utilizes the anthraquinone process, wherein the anthraquinone is first hydrogenated to hydroanthraquinone and then autoxidized to release hydrogen peroxide and the anthraquinone for recycle. Hydrogen peroxide is generated at low concentrations in the solution, and very large flows of anthraquinone and anthrahydroquinone must be handled in order to produce the desired hydrogen peroxide product. Accordingly, such conventionally produced hydrogen peroxide is generally too expensive for commercial use as an oxidizing agent for selective chemical oxidation processes.
An important alternative is generating hydrogen peroxide directly by the catalytic reaction of hydrogen and oxygen, which avoids the difficulty of accompanying large flows of a working solution and can reduce the cost of hydrogen peroxide. The prior art includes a number of catalytic technologies which directly convert hydrogen and oxygen to hydrogen peroxide, but generally utilize a hydrogen/oxygen feed wherein the hydrogen concentration is greater than about 10 mol %, which is well above the flammability limit of xcx9c5 mol % for such mixtures and creates a serious process hazard. At hydrogen feed concentrations below 5 mol %, the prior art catalysts are not sufficiently active and selective to generate hydrogen peroxide product at a reasonable rate. However, an improved process for direct catalytic production of hydrogen peroxide utilizing an active supported phase-controlled noble metal catalyst is now available as disclosed by our co-pending patent application Ser. No. 09/140,265.
Various oxidation processes for organic chemical feedstocks utilizing hydrogen peroxide are known. For example, U.S. Pat. No. 4,701,428 discloses hydroxylation of aromatic compounds and epoxidation of olefins such as propylene using a titanium silicalite catalyst. Also, U.S. Pat. Nos. 4,824,976; 4,937,216; 5,166,372; 5,214,168; and 5,912,367 all disclose epoxidation of various olefins including propylene using titanium silicalite catalyst. European Patent No. 978 316 A1 to Enichem describes a process for making propylene oxide, including a first step for direct synthesis of hydrogen peroxide using a Pd catalyst, and a second step for epoxidation of propylene to form propylene oxide using titanium silicalite (TS-1) catalyst. However, the best hydrogen peroxide product selectivity reported is only 86%, based on the amount of hydrogen converted. In the process second step, the best selectivity of propylene oxide formation is 97%, based on hydrogen peroxide conversion. Therefore, the best overall yield of propylene oxide, based on hydrogen feed, that can be achieved is 83%. However, higher yields of oxidized organic products are much desired, particularly when considering the relatively high cost of the hydrogen feedstock which is required for the direct catalytic synthesis of hydrogen peroxide.
This invention provides a catalytic process which includes two basic chemical transformation reactions or steps for selective oxidation of organic chemical feedstocks with hydrogen peroxide intermediate to produce desired oxidized organic chemical products. In the first reaction step, hydrogen and oxygen are directly catalytically reacted with a suitable solvent such as alcohol/water solutions to form the hydrogen peroxide intermediate by advantageously utilizing a new supported phase-controlled noble metal catalyst which has high reactivity and selectivity. For the catalyst, the noble metal constituent is present as nano-size particles with controlled phase exposition, thereby ensuring that only the most active and selective noble metal sites are available for reaction. In the second reaction step, the hydrogen peroxide intermediate and suitable solvent such as water and/or an alcohol reacts oxidatively with the selected organic chemical feedstock for producing the desired oxidized organic chemical product, with water typically being the main by-product. This second reaction step may be non-catalytic, but will typically utilize a suitable selective oxidation catalyst of either homogeneous or heterogeneous type. The two successive reaction steps are performed in separate catalytic reactors, with intermediate recovery steps being provided as desired for the overall selective oxidative process.
A variety of organic chemical feedstocks can be used in this overall process to produce the desired oxidized organic chemical products. Major classes of organic chemical feedstocks include aromatics, alkanes, ketones and olefins, as well as compounds containing mixed functionality and heteroatoms such as sulfur or nitrogen, with the olefin feedstocks usually being preferred. The major groups of oxidized organic chemical products are alcohols, epoxides, carboxylic acids, hydroxylated aromatics, aldehydes/ketones, glycols, oximes, and N-oxides.
A significant advantage of the present two-step catalytic selection oxidation process utilizing hydrogen peroxide intermediate is that it provides increased yields of the desired oxidized products with respect to hydrogen feed. Because of the high cost of hydrogen, it is critical to minimize the hydrogen demand as much as possible. The present process is able to achieve essentially 100% selectivity of the hydrogen peroxide intermediate based on hydrogen usage. Therefore, the integrated two-step process, consisting of a first step in which hydrogen peroxide intermediate is catalytically synthesized by direct reaction of hydrogen and oxygen (at 100% selectivity), and a second step in which the hydrogen peroxide is catalytically reacted with an organic chemical feedstock such as propylene over titanium silicalite (TS-1) catalyst or the like to make propylene oxide (at  greater than 95% selectivity with respect to H2O2), provides a net overall yield of propylene oxide product with respect to hydrogen of over 95%. This is a significant improvement in hydrogen utilization compared to prior art processes, for which the best reported overall selectivity to propylene oxide product is only about 83%.